(1) Field
The disclosed methods and systems relate generally to optimizing ion implantation uniformity control.
(2) Description of Relevant Art
Ion implantation is a technique for introducing impurities into semiconductor materials to alter the electrical properties of these materials. The impurity material is ionized in an ion source and accelerated to form an ion beam of prescribed energy. The ion beam is then directed at the surface of a semiconductor material such as a silicon wafer. The ions penetrate the wafer and become imbedded into the crystalline lattice of the wafer to form a region of desired conductivity.
Ion implantation systems usually include an ion source for converting a gas or solid material into a well-defined ion beam. The ion beam is mass analyzed to eliminate undesired ion species, is accelerated to a desired energy and is directed onto a target plane. The beam is distributed over the target area by beam scanning, by target movement or by a combination of beam scanning and target movement.
A well-known trend in the semiconductor industry is toward smaller, higher speed devices. In particular, both the lateral dimensions and the depths of features in semiconductor devices are decreasing. State of the art semiconductor devices require junction depths less than 1000 angstroms and may eventually require junction depths on the order of 200 angstroms or less.
The implanted depth of the dopant material is determined, at least in part, by the energy of the ions implanted into the semiconductor wafer. Shallow junctions are obtained with low implant energies. Ion implanters are typically designed for efficient operation at relatively high implant energies, for example in the range of 50 keV to 400 keV, and may not function efficiently at the energies required for shallow junction implantation. At low implant energies, the current delivered to the wafer is much lower than desired and in some cases may be near zero. As a result, extremely long implant times are required to achieve a specified dose, and throughput is adversely affected. Such reduction in throughput increases fabrication cost and is unacceptable to semiconductor device manufacturers. Therefore, an ion implantation system is desired for optimizing the implantation of doses into semiconductor wafers.